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You can use any good oscillator with an added varicap for FM modulation. You can couple the varicap to the resonant circuit (usually at collector) by a 0.5 pF capacitor, and adjust varicap bias and modulation-signal amplitude to obtain the modulation index you need.
Due to the inherent nature and characteristics of FSK, the circuits tend to be more complex than either ASK or OOK.
I'm afraid, the simplest route will be most likely to use a single chip solution like those I post above.
This is one of the reasons there are an abundance of very inexpensive ASK/OOK RX/TX modules and not a comparable number of simple/inexpensive FSK RX/TX modules.
While it is possible to design a FSK RX/TX pair from discrete components, their number and total cost would likely exceed the cost of the single chip solution, not to mention the complexity of tuning the units.
Of course my friend there is circuits, but I agree with BigDogGguru that circuits are not simple.
See this material, I hope its useful for You :
CC1101 RF modem + 250mW Amplifier
- data transfer speed: 1.2 to 500 kbps
- frequency: 779-928 MHz
- output power +10dBm, without PA
- 2-FSK, GFSK, MSK, OOK and ASK modulation schemes
- frequency hopping ready
- automatic address match, package protocol, CRC check
Also today on markets you can find small RF modules up to 5km-7km 433MHz/800MHz, but with incresed output power, about 500mA consuption of current, but first check your country laws about radio diffusion regulations and imports regulations.
If You need simple ASK modulation RF modules here is simple Maxim products MAX1472 and MAX1473 :
Hey tpetar you give nice material BIG thanks to you.
But that nice RF modem is over my range, I just need simple FSK module like this, I found that simple FSK modules exist on markets like ASK modules, here is pictures:
I can easy build modules like thise from pictures.
But that nice RF modem is over my range, I just need simple FSK module like this, I found that simple FSK modules exist on markets like ASK modules, here is pictures:
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I can easy build modules like thise from pictures.
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